Potterton boiler control

[pstokes]

Potterton Profile 60e, fanned, balanced flue boiler

PROBLEM - Boiler starts with fan for about 2 seconds then stops then fan starts again then stops. This cycle repeats perhaps four or five times; eventually fan remains on and ignition sequence starts - all is then well until the next time the boiler has to start up again.
I suspect a sensor somewhere - does anybody have any ideas? In the past I have found several poorly soldered joints on the pcb leading to intermittent operation but this present fault does not seem to be related to that. I have tried increasing the water pump speed without any effect.
Philip

 

[namsag]

Cycling like that is usually caused by a faulty PCB on the profile

 

[gavinda]

agreed.

sometimes when i come across this though, i replace the board and the fault continues when the mains is restored. turn it on and off again and the fault clears.... odd eh?


also check the operation of the air pressure switch too as if this is getting sticky this can give the same fault.

 

[Agile]

Those PCBs sometimes seem to take a short while to become conditioned when first fitted.

I have always thought it was the electrolytics being reformed.

Tony

 

[pstokes]

Thanks for all replys. I'd like to investigate the air valve. I see that there are two air pipes attached to, it Red & Clear. What is the simplest way to check its operation - can I suck or blow on the inlets? I measured the voltages on the microswitch contacts and they appear to be change correctly as the cycling takes place.

 

[sooey]

I have always thought it was the electrolytics being reformed.

Tony please explain further if you don't mind. Sounds like it might be handy to know, even if it isn't it would still be nice.

 

[gavinda]

he could, but then he'd have to kill ya.

 

[sooey]

Don't worry, for I too am a bearer of the secret knowledge but not this bit.

 

[Agile]

I dont give detailed advice on air pressure switches because they are a safety device which should normally only be worked on by a gas qualified person.

What I will say is that blowing into an APS is likely to damage it !

Sooey, an electrolytic capacitor is made from thin aluminium foil sheets seperated by a thin film impregnated with a chemical. The capacitor is "formed" by applying a voltage across the two poles ( or connections ) and this electrolyses the chemical and makes a thin layer of gas as a thin oxide layer on the surface of the foil.

The capacitance is created between the two foils seperated by this thin layer. Because its so thin and made by the action of the polarising voltage on the electrolyte it will gradually degrade if there is no polarising voltage to keep it formed. Furthermore electrolytic capacitors are only normally suitable for use on circuits where there is a DC voltage to keep them formed.

When no voltage has been applied to the capacitor for a while, say many months or a few years, two things happen. The leakage current increases from say 1 microamp to perhaps 300 microamps. The capacitance also increases. These changes in characteristics can cause a change in operation of the circuit for a few minutes when they are first powered. This is more likely to be the case when the manufacture date was a few years before. Normally they largely reform within a few minutes.

I have sometimes noticed this effect on the 407677 range of PCBs as well as some of the ignition circuits which attach to gas valves where it can take a minute or two to start to work normally.

Tony

 

[gavinda]

i thought you were taking the ****.

thats a great answer and does indeed explain why when i fit these boards i sometimes experience this. thanks

gav

 

[Agile]

I thought you were taking the p**s.

gav

Now would I ever do that?

Tony

 

[sooey]

Nice one Tony I agree, great answer, thank you.

 

[pstokes]

I dont give detailed advice on air pressure switches because they are a safety device which should normally only be worked on by a gas qualified person.

What I will say is that blowing into an APS is likely to damage it !
Tony

Since the microswitch is switching properly during the short on/off cycling I now realise the fault is probably with the PCB electronics as was first suggested. Can someone help me find a schematic diagram of the PCB? I have an electronic knowledge and would like to have a go at repairing the fault.

 

[pstokes]

Can someone help me find a schematic diagram of the PCB?.

sorry - should have said 'electronic circuit diagram'.

 

[Agile]

Circuit diagrams are not available for boiler PCBs as the manufacturers dont want you to repair them.

However, that one has only two relays, three transistors and three electrolytics so I expect you could trace it out in an hour or so.

However, you could replace the electrolytics first and if that does not cure the problem then replace the relays.

Tony